The invention concerns a method of purifying uraniferous acid aqueous solutions which contain impurities, the impurities particularly including at least one of the elements zirconium and/or hafnium and at least one of the anions SO.sub.4.sup.=, NO.sub.3.sup.-, Cl.sup.- or F.sup.-. The purification is achieved through precipitation of said impurities by means of an alkaline agent.
Known prior art methods involved with uraniferous ores containing impurities such as zirconium and/or hafnium lead to the production of impure uraniferous concentrates containing from 0.5 to 6% of zirconium relative to the uranium. Although up to a recent date such a quantity of impurities was tolerated by the industries concerned with treating and purifying these concentrates, these industries are at present far more demanding, firstly because of the great difficulties experienced during the purifying treatments, and secondly because of the purity requirements on the part of users. As a matter of fact a standard has been published since 1981, defining the purity of uraniferous concentrates and putting the threshold of impurities for zirconium at less than 0.01% by weight relative to the uranium (ASTM standard No. C967-81).
Such a low impurity threshold for zirconium cannot be obtained by the methods known and described in the literature since, as already mentioned, such methods result in an unacceptable content of impurities.
A uranium upgrading treatment may be applied to uraniferous aqueous solutions resulting, for example, from action on a uranium containing ore, leading to the production of a uranate such as described in "The extractive Metallurgy of Uranium" by R. Merrit, Edition 1971--Colorado School of Mines Research Institute. The product obtained is an impure uraniferous concentrate, since the processing comprises a single stage of precipitation of the uranium by means of an alkaline agent. This gives the uraniferous medium treated a pH of at least 6 and preferably above that level, while bringing about simultaneous precipitation of the uranium and impurities such as zirconium and/or hafnium.
A uranium upgrading treatment may also be applied to these same uraniferous aqueous solutions, leading to precipitation of uranium peroxide, as described in R. Merrit, edition 1971, pages 247 and 248, and in "Proceeding of the International Conference on the peaceful uses of Atomic Energy" United Nations, Vol. VIII, pp. 141 to 143 (1955) by E. L. Zimmer on "Preparation and Separation of Uranium Peroxide, as a stage in the chemical purification of Crude Uraniferous Products". These articles mention that, although precipitation of uranium in the impure solution containing it theoretically enables the impurities present to be kept in solution, the uraniferous precipitate obtained retains not inconsiderable quantities of impurities at the time when the said precipitate is separated. This happens despite consistent and extended washing; thus, precipitation and washing of the uranium peroxide provide a satisfactory way of removing impurities such as silver, arsenic, aluminum, boron, bismuth, calcium, magnesium, manganese, molybdenum, vanadium, sodium, nickel, antimony, tin and zinc, but do not result in adequate elimination of iron, phosphorus and silicon, unless other purifying treatments are carried out at a preliminary stage.
When the uraniferous solution also contains other impurities such as zirconium and/or hafnium, these are generally precipitated simultaneously with the uranium, during the hydrogen peroxide treatment of the aqueous solutions containing them. The resultant product is a uranium peroxide which does not subsequently provide a pure enough uranium for nuclear applications.
The specialized literature also proposes zirconium-uranium separating methods starting from aqueous solutions containing these two elements. Such methods could be used for purification treatment of uraniferous liquors containing zirconium and/or hafnium among other impurities.
One of these processes is described in Czech Pat. No. 187,528 and comprises crystallizing a tetrahydrated zirconium sulfate with a concentrated solution of sulfuric acid, within the range from 30% to 50% by weight. However, such a process has disadvantages which mean that it is of little use industrially: on the one hand it keeps a relatively large quantity of zirconium in solution (of the order of 10% by weight of the initial zirconium) and this is subsequently precipitated with the uranium. Secondly, it requires an extremely large quantity of alkaline agent to neutralize the free acidity and precipitate the uranium at a pH of over 6.
Another of these processes, described in U.S. Pat. No. 4,330,509, claims precipitation of zirconium by means of tartaric acid or ammonium tartrate in a zirconiferous acid liquor where the pH is kept within the range from 0.2 to 1. If such a process were applied to an acid uraniferous solution containing zirconium and/or hafnium among other impurities, the zirconium would be precipitated, simultaneously leading to the precipitation of a large quantity of uranium, which would represent an unacceptable loss for an industrial process.
Thus, the prior art offers unsatisfactory solutions for treatment of a uraniferous liquor containing zirconium and/or hafnium among the impurities present, since the treatments proposed lead to simultaneous precipitation of uranium and at least some of the above-mentioned specific impurities. This necessitates a purification treatment to obtain the purity required in nuclear applications.